A phase locked loop (PLL) is a control system configured to generate an output signal whose phase is related to a phase of a reference signal. PLLs are used in demodulator systems, tone detectors, and frequency synthesizers. PLLs are also used in digital applications which include a high frequency period signal to synchronize events within a circuit.
PLLs include a voltage controlled oscillator (VCO) configured to adjust a frequency of the output signal based on a control signal. In some instances, the VCO includes a varactor. A varactor is a diode having a variable capacitance. In some instances, a metal-oxide-semiconductor (MOS) varactor is used in the VCO. A transmission-line-based inductor is also included in the VCO for high frequency applications, e.g., millimeter-wave region, in some instances.
A Q factor is a measure of an amount of energy loss relative to the energy stored in a resonator, such as the varactor. As the Q factor decreases, the oscillations in the varactor are damped more quickly. In some instances, if the Q factor is too low, the varactor cannot initiate oscillation in the VCO, which inhibits the PLL from locking the output signal to the reference signal. As a frequency of the reference signal increases, the Q factor of the MOS varactor decreases. This decrease potentially prevents initiation of oscillation in high frequency applications.